Charged-particle sources are used for various surface modification, etching, and deposition applications, and are particularly advantageous compared to other methods for providing direct control of particle energy and flux, angle of incidence to the target, and isolation of the target substrate, if different from the conditions of the reactor used to generate the etching and or depositing species.
Broad-beam ion sources have numerous applications in microelectronics device fabrication. Ion beam equipment is used, for example, in the production of multilayer optical coatings, thin-film magnetic heads and in various areas of semiconductor and optical device manufacture.
Charged-particle sources are particularly advantageous for etching and/or deposition of very thin high-quality films over a large uniform area. This is an application of increasing importance as semiconductor and magnetic memory devices advance to higher capacities and device geometries shrink.
An example of a critical large area thin film deposition process is the production of an extreme ultraviolet (EUV) reticles for use in manufacturing photomasks for ultra large scaled integrated circuits. This application requires deposition of a thin film stack of over 80 layers of alternating elements A and B having contrasting indices of refraction, such as Mo and Si, each layer consisting of a few atomic layers of element A or element B. The optical quality of the multilayer reflector is critical. Therefore the thickness of each elemental layer must be very well controlled and repeatable from the first layer to the last to within 0.01 nm. Also, the avoidance of atomic contamination and particulate defects is crucial.
An example of a challenging etching process is the patterning of giant magnetoresistive (GMR) read elements in the wafer processing stage of manufacture of a GMR thin film magnetic recording head or magnetic memory circuit. Such devices are composed of at least several different layers of magnetic and nonmagnetic materials, such as Co and Cu. The performance of the device, i.e. its magnetoresistance, is sensitive to a difference of less than one atomic layer of thickness. To pattern the device without damage requires control of the etch rate of the material to less than 10 nm/min.
To illustrate the level of control of process time which is required, consider deposition of a 2.5 nm film at a deposition rate of 0.1 nm/sec. The deposition time is 25 seconds. Therefore a control error of +/-1 second causes a variation of 2/25=4% in the film thickness. For the EUV mirror application described above, this is unacceptable.
The processes described above are currently being carried out mainly by direct plasma and ion beam deposition and etching techniques. The advantages of ion beam and other charged-particle sources for such applications include the ability to isolate the substrate from plasma-generated particles, provide a controlled and measurable dose of sputtered particles, and to maintain the substrate at a low pressure, for higher film purity in the process chamber where the substrate is located.
One limitation of charged-particle sources of the prior art in these applications is the lack of precise control in the starting, stopping, and duration of the charged-particle flux and particle energies. Source turn-on and adjustment of the discharge power to the desired value requires a few seconds, turn-off requires at least a few hundred milliseconds ramp-down, and the duration of the process itself may very by at least 1 second due to slightly irreproducible software loops in conventional computer-automated control equipment. During the turn-on and turn-off periods the particle energy and flux on the target are not fully under control, which can lead to irreproducible process results. These transition problems can be prevented to some extent by interposing a mechanical shutter between the charged-particle source and the target. The operation sequence with a mechanical shutter may be described as follows:
input the desired preset process time through the software; PA1 turn on the source (with mechanical shutter closed) allow beam to stabilize (e.g. by monitoring the "beam current" on the beam power supply); PA1 open the mechanical shutter, start timer; PA1 when the elapsed time reaches the preset process time, close the mechanical shutter and/or shut down the power supplies. PA1 input the desired preset process time through the software; PA1 turn on the discharge power to the source with the beam voltage off or, preferably, set to a programmed input of 0; allow the discharge to stabilize; PA1 turn "on" the beam voltage, starting extraction of the charged-particle beam; start counting the time; PA1 when the count reaches the preset process time, turn "off" the beam voltage, stopping extraction of the charged-particle beam.
Although the use of a mechanical shutter can reduce the variation in the beam current and energy on the target during the start-up process as compared to simply turning the power supplies on and off, it does not reduce the error in controlling the duration of the process time, and introduces other disadvantages. These disadvantages include:
1. Mechanical shutters in deposition or etch systems build up contamination on the surface, e.g. from sputtered material coming off of the target, and during the shutter motion may shed this contamination on the target or substrate. This can be minimized by slowing the acceleration or deceleration of the shutter, but at the possible expense of etch or deposition uniformity, as described below.
2. Mechanical shutters typically require 1-3 seconds or more to move from a fully open to a fully closed position or vice versa; during the transition period, for a target shutter for example, some portion of the charged-particle flux still reaches the target. This will change the distribution of the charged-particle beam on the target for the transition period and, for very short processes, could cause nonuniformity of deposition or etching on the target. Also during this period, the charged-particle beam is impinging on the edge of the shutter, which could cause contamination of the target by material from the edge of the shutter. All these effects can be reduced somewhat by using high-speed and high-acceleration/deceleration shutters, however at the cost of generating higher levels of particulates in the chamber and perhaps on the substrate (see above discussion).
3. During the time that the mechanical shutter is closed and the source is on, the charged-particle beam is directly impinging on the shutter; this will result in material being sputtered from the shutter and being deposited inside the source and onto the ion optic assembly. This can result in more frequent required maintenance of the source and increased particulate contamination from flaking of deposited material.
4. There is typically a higher pressure in the ion source with the mechanical shutter closed than with it open, since most of the process gas is introduced directly into the source while the vacuum pump is located on the process chamber outside of the source. When the shutter opens, the pressure change can cause a fluctuation in the ion beam current.
5. The incorporation of a mechanical control assembly in the process system involves significant cost and dependence on such a shutter reduces reliability; mechanical shutters are difficult to maintain in a vacuum system, particularly in etch or deposition systems where deposits can build up between moving parts and cause shutter positioning failure or motor damage.